U.S. Republican presidential hopeful Mitt Romney\'s campaign on Thursday stepped up attack on former House speaker Newt Gingrich, whose poll number has been rising recently as the Iowa caucuses approach.In a conference call with reporters, top Romney supporters attacked Gingrich\'s leadership and record. Former Senator from Missouri Jim Talent said Gingrich is \"not a reliable and trusted conservative leader.\"\"If the nominee is Newt Gingrich, the election is going to be about the Republican nominee, which is exactly what President Obama and the Democrats want,\" said Talent.Former New Hampshire Governor John Sununu also highlighted Gingrich\'s \"irrational behavior,\" and \"anti-principled actions,\" which he said could compromise a commander in chief.\"Gingrich\'s undercutting of Paul Ryan proves that he\'s more concerned about Newt Gingrich than he is about conservative principles,\" said Sununu.New poll results suggest Gingrich is now leading Romney in the GOP field, whereas the former front-runner\'s numbers stood stagnant for months. The development is forcing Romney campaign to focus more on Gingrich and view him as a serious threat.